My long-term objective is to use the genetically tractable eukaryote Saccharomyces cerevisiae to determine the molecular mechanisms underlying the establishment and maintenance of cell polarity. More specifically y, I am interested in understanding how Cdc42p, an evolutionarily conserved small GTPase, controls the organization of the actin cytoskeleton and of the septins through independent pathways. Previously, we have shown that Msb3p and Msb4p, a pair of highly homologous proteins in yeast, function as dosage-dependent suppressors of cdc42 mutations and that both proteins also regulate secretion by functioning as GTPase-activating proteins (GAPs) for the Rab GTPase Sec4p; thus, defining a molecular linkage between Cdc42p-mediated signaling pathway and Sec4p-mediated secretion. Recently, we found that Msb3p and Msb4p bind directly to Cdc42p-GDP and also to Spa2p, a scaffold protein of the "polarisome" that also includes the formin Bnilp and the actin-binding protein Bud6p; thus, defining a concrete pathway from Cdc42p to actin organization. Currently, we are trying to determine the detailed biochemical mechanisms underlying the function of the polarisome, the components of which are evolutionarily conserved. Another line of our recent work has led to a two-step model on the septin-ring formation at the beginning of the cell cycle: the recruitment of septins to the incipient bud site and the assembly of the recruited septins into a cortical ring. Cdc42p plays a role in both steps. With this framework, we are trying to define the role of Axl2p, a type I membrane, and Rgalp, a Cdc42p GAP, in septin organization. Finally, we have isolated PXL1 as a multicopy suppressor of several cdc42-Ts alleles. Pxllp shows extensive similarity in amino-acid sequence and structural organization with paxillin, a focal adhesion protein that is essential for development and cell motility. We are using Pxllp as a model to understand how this family of proteins is targeted to discrete cellular locations through their C-terminal LIM domains, a crucial gap in paxillin biology. Homologs of Cdc42p are involved in diverse cellular processes, such as cell polarity, cell migration, and cell growth control. In addition, deregulation of Cdc42p activity in mammals is associated with serious diseases, such as cancer. Thus, studying the signaling mechanisms of Cdc42p in yeast will have profound implications in basic biology and clinical sciences. [unreadable] [unreadable]